1. The Field of the Invention
This invention relates to a method for removal of selenium from water. More particularly, this invention relates to the use of microbes for use in removal of selenium from water at a particular location containing selenium contamination.
2. Technical Background
Selenium is the thirtieth (30th) most abundant element. Selenium is a naturally occurring metalloid element having atomic number 34 and an atomic weight of 78.96. It lies between sulfur and tellurium in Group 16 and between arsenic and bromine in Period 4 of the periodic table of elements. Selenium is widely dispersed in igneous rock. In hydrothermal deposits, it is associated isothermally with silver, gold, antimony, and mercury. Selenium also appears in large quantities, but in low concentrations, in sulfide and porphyry copper deposits. Moreover selenium is widely associated with various types of sedimentary rock.
Inorganic selenium is most commonly found in four oxidation states (Se.sup.6+, Se.sup.4+, Se.sup.0, and Se.sup.2-). Selenate (SeO.sub.4.sup.2-) and selenite (SeO.sub.3.sup.2-) are highly water soluble. Elemental selenium (Se.sup.0) is much less soluble in water. The most reduced form, hydrogen selenide (H.sub.2 Se) occurs as a toxic gas, but is readily oxidized to elemental selenium in the presence of air.
Elemental selenium can be oxidized to selenite by microorganisms or chemically oxidized in alkaline or mildly acidic conditions. Further oxidation of selenite results in the conversion to selenate. In highly oxidizing environments, such as well-aerated surface waters, especially those with alkaline conditions, the majority of selenium is present as selenate. The relative proportions of selenite and selenate depend on water redox potentials and pH. Selenite is reduced to elemental selenium under mildly reducing conditions while selenate reduction occurs under stronger reducing conditions.
Selenium forms covalent compounds with most other substances and is necessary in small amounts for most forms of life. Selenium is a chemical analog of sulfur and can interfere with normal cellular metabolism. Selenium is a teratogen in mammals and birds, contributing to birth defects in several species. Data related to selenium poisoning has been extensively published over the last decade. Selenium has been reported to cause death and/or mutations in fish and waterfowl. A highly publicized case of selenium poisoning occurred in California's Kesterson Wildlife Reservoir, resulting in its closure.
Selenium is a common water contaminant throughout the United States and the world and represents a major environmental problem in at least nine western states. Selenium contamination originates from many sources including mining operations, mineral processing, abandoned mine sites, petroleum processing, and agricultural run-off.
The principal sources of selenium contamination in mining are copper and uranium bearing ores and sulfur deposits. Selenium is commonly found in these mining wastewaters in concentrations ranging from a few micrograms per liter up to more than 12 mg/L. In precious metals operations, waste and process water and heap leachate solutions selenium may be present at concentrations up to 30 mg/L. While most of these mining operations are currently zero-discharge facilities, eventual treatment will be necessary to remove selenium to meet discharge and closure requirements.
Additionally, in many areas in the Western United States, natural background selenium concentration in irrigation drainwater and streams may exceed 2.0 .mu.g/L. Selenium in irrigation drainwater has been a concern for the past two decades because of its adverse effect on aquatic life and waterfowl.
The National Primary Drinking Water Standard maximum containment level (MCL) for selenium is 50 .mu.g/L and the maximum contaminant level goal (MCLG) is 50 .mu.g/L. The current EPA National Ambient Water Quality Criteria, adopted as standards by most Western States for protection of aquatic life is 5.0 .mu.g/L (chronic) and 20.0 .mu.g/L (acute). However the U.S. Fish and Wildlife Service has suggested that these standards are inadequate, particularly with regard to protection of fish, waterfowl, and endangered aquatic species. Questioning of these standards has arisen because some laboratory and field studies indicate that water borne selenium concentrations as low as 2.0 .mu.g/L may bio-accumulate in complex aquatic food chains to toxic levels.
Removal of selenium from large volume waters to meet discharge criteria is one of the nation's more complex environmental problems. Several methods have been investigated for treating agricultural wastewaters. These treatments generally involve the reduction of SeO.sub.4.sup.-2 to elemental selenium using either chemical reduction with ferrous hydroxide [Fe(OH).sub.2 ] or biological reduction using an indigenous bacteria. The chemical reduction method requires operating conditions that can be difficult to obtain in large treatment operations such as a pH of about 9 and a large excess of Fe(OH).sub.2. This method has proved to be too costly to sustain on a large scale.
Other treatment processes, such as lime precipitation, chemical reduction, activated alumina adsorption, ion exchange, reverse osmosis, electrodialysis, or distillation have been demonstrated to remove a variety of pollutants including selenium from water to below drinking water standards. However, these methods are generally expensive and cost prohibitive for the treatment of large volumes of water.
Moreover, lime precipitation and chemical reduction processes can result in a mixed metal waste product that can increase treatment and disposal costs, and generally require some pH adjustment of the influent. Additionally, laboratory tests and pilot plant studies have shown that chemical precipitation employing alum, lime, ferrous sulfate or ferric sulfate are substantially ineffective for removing selenium in the selenate oxidation state.
A process for the removal of selenium from waters using a reactor containing microbial biomass has been investigated. However, this method is inadequate because it is intended to treat water that contains at least 0.5 mg/L of selenium and is only capable of lowering the selenium concentration to 10% of the original value. In order to achieve the drinking water standard, a polishing step such as ion exchange is needed. Additionally, this method is inhibited by anions such as nitrate and sulfate.
Generally, selenium containing waters greatly differ from each other in type of selenium contamination, levels of selenium, types of co-contaminants, levels of co-contaminants, pH, ionic content, etc. These factors make the bioremediation of selenium containing waters complex and some what site-specific.
From the foregoing it will be appreciated that it would be a significant advancement in the art to provide a low-cost method of removal of selenium contamination from water. It would be a further advancement to provide a method that would remove selenium contamination to below the MCL. It would be an additional advancement to provide a method that would economically remove selenium concentrations to below drinking water standards. It would be an further advancement in the art to provide a method that could remove both selenate and selenite. It would be a further advancement to provide a method that could remove selenium in concentrations ranging from below 0.5 mg/L to greater than 100 mg/L. It would be a further advancement to provide a method of bioremediation that could remove selenium from widely varying waters. It would be a further advancement in the art to provide a method of bioremediation that could remove selenium from widely varying waters using retention times ranging from 4+.24 hours.
Such methods of selenium removal are disclosed herein.